Japanese Patent Application Laid-Open Publication No. 2004-45304 discloses a technique of measuring a distance to an object located in a target space by using intensity-modulated light. When using the intensity-modulated light of a sinusoidal waveform, the light reflected from the object is also of a sinusoidal waveform with a phase difference that varies with a distance to the object. Thus, the distance to the object in the illuminated target space can be measured based upon the phase difference between the emitted and reflected intensity-modulated lights.
Based upon an intensity measurement of the received intensity-modulated light for each of a plurality of phase ranges, the phase difference can be derived from a relation between the location of the phase range and the received light intensity. For instance, the received light intensity lr can be expressed to be lr=η·l(t−d)+le, wherein l(t) denotes the directed light intensity as a function of time t, η is a light attenuation factor due to a distance to the object or reflectance thereat, le is an intensity of environmental (disturbance) illumination, and d is a delay time corresponding to the distance L to the object and is expressed by d=2L/c.
The above expression has three unknowns of the attenuation factor η, the delay time d, and the environment illumination intensity le, which can be obtained through measurements of received light intensities respectively at three or more different times. Thus, it is possible to obtain the distance to and reflectance of the object as the spatial information. Since the intensity-modulated light is generally designed to cyclically vary its intensity, the integration of the received light intensities over a plurality of periods of cycle can restrain influence of fluctuation in the environmental illumination or noise developed in the device.
In order to detect the spatial information with the above technique, it is necessary to precisely interrelate the phase ranges of the intensity-modulated light directed to the target space with those of the received light. There are general techniques for extracting electric charges from the light-receiving element at a specific one of phase ranges, one sending a signal designating the specific phase regions for extraction of the electric charges from the light-receiving element with a control electrode provided for control of the timing of extracting the charges (as in the light-receiving element made of CCD or the like), and the other selecting only the electric charges extracted in the period corresponding to the specified phase region (as in the light-receiving element made of photo-diode or the like). In order to improve accuracy of the measurements, either of the above techniques necessitates to precisely synchronize the signals one given to the light-emitting element with the other given to the light-receiving element for the electric charge extraction.
However, the light-emitting element, the light-receiving element, as well as circuits for generating the signals associated with these elements may suffer from variations in their characteristics depending upon varying ambient temperature and humidity. Thus, there is always a likelihood of exaggerating an error in the measurement results due to the environmental changes even operating the device after calibration thereof.